1. Field of the Invention
The present invention relates to liquid crystal display devices. More particularly it relates to liquid crystal displays having a ferroelectric liquid crystal polymer orientation film.
2. Discussion of the Related Art
A liquid crystal display device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Liquid crystal molecules have a definite orientational alignment as a result of their long, thin shapes. That orientational alignment can be controlled by an applied electric field. In other words, as an applied electric field changes, so does the alignment of the liquid crystal molecules. Due to the optical anisotropy, the refraction of incident light depends on the orientational alignment of the liquid crystal molecules. Thus, by properly controlling an applied electric field a desired light image can be produced.
While various types of liquid crystal display devices are known, active matrix LCDs having thin film transistors and pixel electrodes arranged in a matrix are probably the most common. This is because such active matrix LCDs can produce high quality images at reasonable cost.
FIG. 1 is a cross-sectional view illustrating a conventional twisted nematic (TN) LCD cell in an active matrix LCD. As shown, the TN-LCD cell has upper and lower substrates 1a and 1b and an interposed TN-LC layer “LC”. The lower substrate 1b has a TFT (“TFT”) as a switching element that switches a voltage that changes the orientation of the LC molecules. The lower substrate 1b also includes a pixel electrode 115 that is used to apply an electric field across the LC layer in response to signals applied to the TFT. The upper substrate 1a has a color filter 25 for producing a color, and a common electrode 114 on the color filter 25. The common electrode 114 serves as an electrode that produces the electric field across the LC layer (with the assistance of the pixel electrode 115). The pixel electrode 115 is arranged over a pixel portion “P,” i.e., a display area. Further, to prevent leakage of the LC layer, the two substrates 1a and 1b are sealed by a sealant 6.
As described above, since the pixel and common electrodes 115 and 114 of the conventional TN-LCD panel are positioned on the lower and upper substrates 1b and 1a, respectively, the electric field induced between them is perpendicular to the lower and upper substrates. The described liquid crystal display device has advantages of high transmittance and a high aperture ratio. Furthermore, as the common electrode on the upper substrate acts as a ground, the liquid crystal is shielded from static electricity.
When no electric field is applied to the LC, the TN-LC molecules are aligned as shown in FIG. 2. As shown by the arrows, the longitudinal axes of the TN-LC molecules gradually twist along polar angles (along a helical axis) with respect to the substrates such that the TN-LC molecules gradually rotate 90 degrees between the lower substrate 1b and the upper substrate 1a. Also as shown in FIG. 2, first and second polarizers 18 and 30 are positioned on the exterior surfaces of the lower and upper substrates 1b and 1a, respectively. The longitudinal axes of the liquid crystal molecules in contact with the lower substrate 1b align with the axis of the first polarizer 18. Likewise, the longitudinal axes of the liquid crystal molecules in contact with the upper substrate 1a align with the axis of the second polarizer 30.
Without an electric field applied across the LC, when light is incident on the first substrate 1b the portion of that incident light that is polarized in the direction of the first polarizer 18 enters the liquid crystal cell. The liquid crystal LC twists the polarization of the entering light until it reaches the second polarizer 30. Then, the light has the same polarization as the polarization of the second polarizer 30. The light then freely leaves the liquid crystal cell.
Referring now to FIG. 3, when an electric field “E” is applied across the liquid crystal LC, the TN-LC molecules align perpendicular to the upper and lower substrates 1a and 1b. That is to say, the molecular alignment of the TN-LC is set by the perpendicular electric field E such that the longitudinal axes of the TN-LC molecules tend to become parallel with the direction of the electric field E. The rotation of the polarization of the incident light that enters the first polarizer 18 does not take place. The entering light is then blocked by the second polarizer 30.
A TN-LCD that operates according to the foregoing description has a serious disadvantage in that it has a narrow viewing angle. Since the TN-LC molecules are gradually rotated with a gradual change of the polar angle, the contrast ratio and brightness rapidly fluctuate with respect to the viewing angle.
Accordingly, to address the above-mentioned problem, an in-plane switching (IPS) LCD panel has been developed. Unlike the TN (or STN) LCD panel described above, an IPS-LCD panel uses an electric field that is parallel with the substrates.
A more detailed explanation about the operation of a typical IPS-LCD panel will be provided with reference to FIGS. 4 through 8.
As shown in FIG. 4, lower and upper substrates 1a and 1b are spaced apart from each other, and a liquid crystal “LC” is interposed therebetween. The lower and upper substrates 1a and 1b are often referred to as array and color filter substrates, respectively. On the lower substrate 1a are a pixel electrode 15 and a common electrode 14. The pixel and common electrodes 15 and 14 are aligned parallel to each other. On a surface of the upper substrate 1b is a color filter 25 that is positioned between the pixel electrode and the common electrode of the lower substrate 1a. A voltage applied across the pixel and common electrodes 15 and 14 produces an electric field “E” through the liquid crystal “LC.” The liquid crystal “LC” has a negative dielectric anisotropy, and thus it aligns parallel to the electric field “E”.
FIGS. 5 to 8 conceptually help illustrate the operation of a conventional IPS-LCD device. When no electric field is produced by the pixel and common electrodes 15 and 14 (reference FIG. 5), the longitudinal axes of the LC molecules “LC” are parallel. For example, FIG. 6 shows a common angle of 45 degrees between a line that is perpendicular to the pixel and common electrodes 15 and 14 and the longitudinal axes of the LC molecules.
On the contrary, when an electric field is produced by the pixel and common electrodes 15 and 14 (reference FIG. 7), because the pixel and common electrodes 15 and 14 are on the lower substrate 1a, an in-plane electric field “E” that is parallel to the surface of the lower substrate 1a is produced. Accordingly, the LC molecules “LC” twist to bring their longitudinal axes into coincidence with the electric field. Thus, as shown in FIG. 8, the LC molecules align with their longitudinal axes parallel with a line perpendicular to the pixel and common electrodes 15 and 14.
In the above-mentioned IPS-LCD panel, there is no transparent electrode on the color filter. Furthermore, the liquid crystal used in the above-mentioned IPS-LCD panel has a negative dielectric anisotropy.
The IPS-LCD mode has an advantage of a wide viewing angle. Namely, when a user looks at the IPS-LCD display device a wide viewing angle of about 70 degrees in all directions (up, down, right and left) is achieved. Additionally, the IPS-LCD device has low color dispersion and a relatively simple fabrication process.
However, since the pixel and common electrodes are on the same substrate, the transmittance and aperture ratios are low. In addition, the response time to a driving voltage is not optimal. Finally, the color of their images tends to depend on the viewing angle.
FIG. 9 is a graph of the CIE (Commission Internationale de l'Eclairage) color coordinates of a conventional IPS-LCD device that shows the dispersion of color. The horseshoe-shaped area is the distribution range of the wavelength of visible light. The results are measured using a standard white light source [point (0.313, 0.329) in CIE coordinates] and various viewing angles of right, left, up and down, and 45 and 135 degrees. The range of the color dispersion is so long that the white light emitted from the conventional IPS-LCD device is dispersed largely according to the viewing angle. This results from the fact that the operation of the conventional IPS-LCD device is controlled by birefringence.